The Cairo Fossil Forest is a unique collection of 385 million-year-old trees preserved in an abandoned quarry in upstate New York. The forest holds some of the oldest trees in the world, as well as the oldest known examples of trees with leaves and thick, woody trunks, researchers say.

“The Cairo site is very special,” Christopher Berrya paleobotanist at the University of Cardiff in the U.K., told Science Magazine when the discovery was announced in 2019. “Standing on the quarry surface we can reconstruct the living forest around us in our imagination,” Berry, who was one of the scientists that found the site in 2009, added.

The remains of ancient trees are buried beneath the quarry floor, but they form patterns on the surface that reveal where tree trunks and roots once anchored into the ground. Some of the fossilized roots are almost 6 inches (15 centimeters) thick, Science Magazine reported, and the biggest root networks stretch 36 feet (11 meters) away from the trunks.

The trees likely belong to Archaeopterisan extinct group of trees with fern-like leaves that is related to modern trees. Research suggests Archaeopteris trees grew more than 66 feet (20 m) tall toward the end of the Devonian period (419.2 million to 358.9 million years ago), which is also known as the “age of fishes” due to the huge diversification that happened in this animal group.

The first Archaeopteris trees marked a turning point in Earth’s history, because they helped to suck up and lock away carbon dioxide (CO₂) from the air, shifting the composition of the planet’s atmosphere. These plants also accelerated a process called weathering, which occurs when roots jumble and break up rocks, exposing them to air and triggering a chemical reaction that turns CO₂ into carbonate ions. These ions eventually end up in the ocean and bind together to form limestone, experts told Science Magazine.

Before the Cairo Fossil Forest discovery, the oldest found Archaeopteris trees were 365 million years old, Berry said. The discovery of the Cairo Fossil Forest in 2009 suggests these prehistoric trees evolved at least 20 million years earlier than that, but it’s still unclear exactly when they first appeared on Earth.

Related: 23 million-year-old petrified mangrove forest discovered hiding in plain sight in Panama

Despite being the earliest example of leaves and thick trunks appearing in trees, the Cairo Fossil Forest isn’t the oldest in the world. In 2024, scientists discovered 390 million-year-old plant fossils in the southwest of England that dethroned the Cairo Fossil Forest as the earliest known forest. These older fossils belong to an extinct type of plant that looked like palm trees and are thought to be closely related to ferns and horsetails.

“They’ve got a long central stem and what look like palm fronds coming off, but those palm fronds aren’t really leaves — they’re actually just lots of twiglets,” Neil Daviesa professor in the Department of Earth Sciences at the University of Cambridge in the U.K., told Live Science at the time.

Similar plants to those found in southwestern England have been discovered just 25 miles (40 kilometers) away from the Cairo Fossil Forest in Gilboa, New York. The Gilboa Fossil Forest is 382 million years old and was first excavated in the 1800s. It held the title of “world’s oldest forest” until researchers discovered the forest in Cairo.

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